1. Field of the Invention
The present invention relates to a lightning arrestor insulator wherein ZnO elements are embedded and integrally fixed inside the insulator body.
2. Description of the Prior Art
As a means to protect an insulator itself from overcurrent in the case of the striking of a thunderbolt, attention has been drawn to a lightning arrestor insulator, such as shown in Gazettes of Japanese Patent Application Laid-open Nos. 124,294/1979, 32,308/1980, 49,178/1984, etc., wherein an element comprising ZnO as a main component having voltage non-linear resistant characteristics (hereinafter referred to as ZnO element) is embedded in the insulator and integrally fixed with an inorganic bonding material such as cement, glass or the like. Such a ZnO element reacts with moisture in air and gradually reduces its resistance value, with increasing heat generation, whereby the insulator is put in danger of fracture, so that it is usually fixed inside the insulator, with its periphery sealed hermetically so as to prevent from deterioration.
In such a case, for the purpose of avoiding the deterioration by heat of the ZnO element during the sealing, a low melting glass which enables the sealing at low temperatures has been heretofore selected as the inorganic bonding material, without giving any particular consideration to the thermal expansion coefficient of the inorganic bonding material, and so upon a rapid cooling during heat treatment in the manufacturing step or when the lightning arrestor insulator whose temperature has been elevated by applied voltage is subjected to rain, snow or the like, or upon a rapid heating due to a striking thunderbolt, a large thermal stress is formed in the lightning arrestor insulator and cracks initiate on the interface between the inorganic bonding material and the insulator, ZnO element, etc. and, according to circumstances, there have existed possibilities to induce a serious accident such as fracture of the insulator.